Some uncertainty exists as to the real purpose of the CLOCK signal - Pin 2 on the GamePack edge connector. It is actually the processor clock (4.194304 MHz) divided by four (1.048575MHz).  However it serves a much more important role as it indicates when the address and data lines are stable when interfacing to external IO devices.  For those of you that remember the Motorola 6809 this used to be called the 'E line'.  The timing diagram bellow shows that when the Pin 2 is low the address and data lines are valid.
 

If the enable of an IO device uses just the Address and /RD lines and Pin 2 is not included then the device will be frequently enabled for short periods as the address lines stabilise. In many situations this may not be a problem but you should be aware of this condition.